1.Field of the Invention
The present invention relates to a light source module, and more particularly to a back light module.
2.Description of Related Art
In recent years, back light modules in liquid crystal displays (LCD) mostly adopt light emitting diodes (LED's) that have features such as long life, high efficiency, and low pollution to the environment. Brightness of LED's relates to display quality of an LCD. Therefore, today's manufacturing technology emphasizes on the design of back light modules.
FIG. 1 is a circuit block diagram of a conventional back light module. Referring to FIG. 1, a conventional back light module 100 uses an output voltage Vout generated by a voltage converter 110 to drive an LED matrix 120, which comprises a plurality of sets of LED series. A current adjusting circuit 130 is used to provide a current that flows through the LED matrix 120. In addition, the current adjusting circuit 130 controls turn-on status of its internal switches SW11˜SW14 so as to change an average current of each set of the LED series provided by current sources 131˜134 at a predetermined time. Accordingly, the current adjusting circuit 130 may adjust a brightness level of a light source generated by the LED matrix 120 by controlling the switches SW11˜SW14.
In another aspect, the voltage converter 110, the LED matrix 120, and a feedback compensation circuit 140 comprise a closed loop. An error amplifier 141 compares feedback voltages Vfb1˜Vfb4 generated by each set of the LED series with a reference voltage Vref, and a voltage controller 142 generates a control signal Sct according to the comparison result from the error amplifier 141. Accordingly, the voltage converter 110 adjusts a level the output voltage Vout based on the control signal Sct.
However, in practical applications, in the conventional back light module 100, the current of each set of the LED series is controlled by a switch and a current source so when contrast of a display image in an area control is raised, the number of the switches and the current sources in the current adjusting circuit 130 of the conventional back light module 100 must be increased in response. In this case, the conventional back light module 100 requires tremendous power consumption. As a result, temperature of internal circuits is increased and lifetime is decreased.